The primary objective is to identify and study genetically-associated variations in the Ammon's Horn (hippocampus and dentate gyrus) of inbred strains of house mice. Further studies of extreme strains and of progeny resulting from selected crosses between them are to be conducted to determine how variations are inherited, what behavioral differences may be associated, and what developmental events may be involved. The variations presently being examined involve differences either in neuron numbers of specific neuronal regions, or in the distribution of mossy fiber boutons upon dendrites of hippocampal neurons of Regio Inferior.